The present invention relates generally to the field of controlling temperature and more particularly to the improvement of temperature dynamic response using dynamic feedforward compensation.
A heater is used to manipulate a temperature of a thermal load in a wide variety of applications. Examples of heaters and respective thermal loads include, without limitation: space heaters for heating ambient air in living spaces; industrial ovens for heating materials in manufacturing processes; and cooking appliances for heating foods in meal preparation. Examples of cooking appliances include, without limitation, gas ranges, electric ranges and radiant electric cooktops.
In many such temperature control applications, a user provides an input command requesting a step change in heater output power and then waits for a corresponding temperature change to occur in the thermal load. The temperature change eventually achieved in steady state depends on the change in heater output power and on an equivalent thermal resistance of the thermal load. Evolution in time of the temperature change depends on one or more system time constants. Thermal system time constants arise, for example, as functions of thermal resistance distribution and thermal capacitance distribution within both the thermal load and the heater. Non-thermal system time constants arise, for example, as functions of any other energy storage mechanisms inherent in the heater and in any apparatus used for controlling the heater.
In some applications, the system time constants are large enough, compared to a desired response time, to warrant use of an automatic control system to attempt to quicken the command response. In a cooking application, for example, after a pan has been at a high temperature setting long enough to initiate boiling, it is desirable to set the temperature to a lower setting for simmering and have the temperature reduce quickly enough to avoid having the pan contents boil over.
Conversely, in some applications, the system time constants are small enough, compared to the desired response time, to warrant use of an automatic control system to attempt to slow the command response. In another cooking application, for example, after a pan has been at a low temperature it is desirable to set the temperature to a higher setting and have the temperature increase slowly enough to maintain an acceptably uniform temperature throughout the pan contents.
Approaches to automatic control system design, useful for either quickening or slowing the command response, divide into two classes: control systems which use temperature measurements (called xe2x80x9cfeedback designsxe2x80x9d, xe2x80x9cclosed loop designsxe2x80x9d, or xe2x80x9cthermostatsxe2x80x9d) and control systems which do not use temperature measurements (called xe2x80x9cfeedforward designsxe2x80x9d, xe2x80x9cdynamic feedforward compensationxe2x80x9d or xe2x80x9copen loop designsxe2x80x9d).
When successful, feedback designs generally enjoy the advantages of being more accurate in response to temperature commands and of being less sensitive in response to external disturbances, such as, for example, ambient temperature and atmospheric pressure, than comparable feedforward designs. However, feedback designs generally suffer the disadvantages of being more expensive, owing to the cost of a temperature sensing device, and of being susceptible to instability in the face of unanticipated thermal load dynamics. Excessive sensitivity to thermal load dynamics results in an unsuccessful feedback design.
In temperature control applications where cost and the ability to accommodate a large assortment of thermal loads take precedence over accuracy and disturbance sensitivity, an opportunity exists for using an open loop design of a temperature controller to modify the temperature command response. For example, surface cooking is a temperature control application in which temperature is typically uncalibrated. Being uncalibrated, the success of a surface cooking application is critically dependent on low cost and the ability to operate properly with a wide range of cooking vessels (e.g., pots and pans) and cooking vessel contents (i.e., foods), while being indifferent to temperature accuracy and disturbance sensitivity.
The opportunity described above for using an open loop design of a temperature controller to modify a temperature command response is addressed by the present invention. In one embodiment of the present invention, an apparatus for controlling a temperature of a thermal load comprises: a dynamic compensator for calculating a heater command as a function of a user input signal without using a temperature measurement; and a heater for controlling said temperature of said thermal load by applying heat in response to said heater command.